Titanium alloys are widely used in aeronautical and aerospace applications and other applications where it is desirable to minimize the weight of mechanical structures, because of the high strength-to-weight ratios that titanium alloys offer. However, in terms of surface characteristics, titanium alloys suffer from some disadvantages. Specifically, titanium alloys are prone to galling and seizing when placed in loaded sliding contact with parts made of either titanium alloy or other metals. Additionally, titanium alloys are prone to oxidation at elevated temperatures.
For at least these reasons, parts made of titanium alloys are frequently plated with chromium when they are to be used in bearings or the like where they will be in sliding contact with other parts. Chromium plating substantially reduces the problem of galling and seizing of titanium parts. Chromium also has good anti-corrosion properties relative to titanium.
Chromium is typically electroplated onto a titanium part. In a conventional electroplating process for plating chromium onto titanium, after the surface of the part to be plated is cleaned and degreased, the surface is activated so that it will be more-receptive to the chromium plating and thus improve the strength of the bond between the titanium substrate and the chromium plating. The activation is conventionally accomplished by immersing the part in a series of chemical solutions. As described in "Electroplating on Titanium Alloys" by Leo Missel, published in the September 1957 issue of "Metal Finishing", Ti6-4 alloy (a titanium alloy containing 6 percent aluminum and 4 percent vanadium) is activated by first immersing the part in a solution of hydrofluoric acid and nitric acid, then immersing the part in a solution of sodium dichromate and hydrofluoric acid. The part is then rinsed with tap water and is transferred to a chromium bath for electroplating.
The activation solutions can be hazardous to workers who perform the process or work near the plating apparatus, and consequently the solutions are frequently subject to regulatory restrictions in terms of their use. The solutions are also prone to being contaminated and thus must periodically be replaced. Accordingly, the contaminated solutions must be disposed of, which poses hazards to the environment. Furthermore, the activation process is time consuming. For example, the aforementioned Missel article recommends immersing the part in the sodium dichromate-hydrofluoric acid solution for 20 minutes.
Accordingly, it would be desirable to provide a method for electroplating chromium onto titanium which does not require the use of hazardous chemicals for activating the titanium surface prior to electroplating the part. Additionally, it would be desirable to provide a chromium electroplating process for titanium which does not require any chemical activation step prior to electroplating the part.